Kinetics of Pb and Pb-Ag anodes for zinc electrowinning .2. Oxygen evolution at high polarization

Electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction and secondary ionisation mass spectroscopy (SIMS) have been utilized to investigate the mechanism of reactions taking place on Pb and Pb-Ag alloy anodes in acidic zinc sulphate electrolytes, in the potential domain above 1.16 V sse. At these polarizations, the main reaction is oxygen evolution on a porous deposit of PbO2, or PbO2 + MnO2 in Mn2+-containing electrolytes. in all cases, the electrode impedance reveals a slow electrode activation with increasing polarization, probably related to the deposit conductivity. With lead-silver anodes it also appears an inhibition process of the main reaction, which becomes more important and turns into passivation, when the electrolyte contains Mn2+ ions. This process has been assigned to the formation of inhibiting adsorbates on the oxide layers.